I would be extremely concerned about that high of a temp being "usual". Someone will surely correct me if I'm wrong, but I have quite a few years of experience with various models of MB, including the SL, and the "normal" operating temp of all of them is 80-85C, and that is running at highway speeds or sitting in traffic in 100F degree temps. I'm cetainly not a tech, but I would look at the thermostat first and then maybe the water pump. It's also possible that the gauge calibration is off. Whatever the cause it needs to be found because that is way to high of a normal running temp.

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1996 SL320 97K miles
1996 C220 130k miles
1992 500SEL 170k miles <---sold
1986 300E 216k miles <---sold